1. Field of the Invention
The present invention relates to improved filler reinforced organosilicic polymer compositions, and, more especially to novel organopolysiloxane compositions comprising a reinforcing amount of certain precipitated silica particulates as reinforcing filler therefor, said silica particulates being characterized both by large BET and CTAB specific surface areas, and the resultant reinforced compositions having improved mechanical and thermal properties upon vulcanization.
2. Description of the Prior Art
It has long been known to this art to reinforce the mechanical and thermomechanical properties of the various organopolysiloxane elastomers by incorporating therein, prior to the vulcanization or curing thereof, various fillers, such as carbon black and precipitated or pyrogenic silica particulates.
And by far the best reinforcing properties have been obtained by using pyrogenic silica particulates having a large specific surface area. To date, though, it has not been possible to duplicate the results obtained with pyrogenic silica particulates when using reinforcing amounts of precipitated silica particulates in said organopolysiloxane elastomers.
Accordingly, this art is replete with reported attempts to improve the reinforcing nature of the precipitated silica particulates to a degree such as would render same virtually identical to the pyrogenic silica particulates in desirable properties, which pyrogenic silicas unfortunately are much too expensive to be of practical worth.
Thus, it has long been attempted, in order to enhance the adaptability of the silica particulates for the purpose of reinforcing organosilicic elastomers, to develop various processes for preparing precipitated silicas, which attempts became increasingly complex and sophisticated, and wherein the temperature, levels of concentration of the reactants (silicate, acid, etc.) the flow rates of the reactants, the sequences of successive reactant addition operations, the pH, etc., had to be very carefully controlled. Compare, for example, French Pat. No. 1,352,354; U.S. Pat. Nos. 3,954,944 and 4,127,641.
Attempts were also made, moreover, to improve the reinforcing nature of precipitated silicas for silicone applications, by rendering the silica particulates hydrophobic by suitable surface treatment (using, for example, a silane, silazane, or like agent). Hydrophilic silicas which have been rendered hydrophobic by means of such a treatment and which can be used as reinforcing fillers for the silicones are described, for example, in French Pat. No. 2,356,596.
Finally, and quite recently, there has also been developed a precipitated silica which is greatly improved filler-wise, but which imparts to organopolysiloxane elastomers, properties of reinforcement which are considerably poorer than those imparted by the pyrogenic silicas (International Rubber Conference held at Kiev, Oct. 10-14, 1978).
Cf. French Pat. Nos. 1,453,198, 1,483,791, 1,150,836, 2,218,285 and 2,353,486; British Pat. No. 883,863.